This invention relates to a power module structure for an electronic component requiring a high level of reliability and heat-radiating capability, as well as to a solid state relay that uses such a power module structure.
As circuits are coming to be integrated in recent years, it is becoming a common practice to use a heat sink to remove heat from a heat-generating electronic component. For this reason, it is becoming an important technological problem to intimately contact an electronic component or a substrate to a heat sink for effectively removing heat. Since solder materials with a high melting point are coming to be used for reducing harmful substances to be used, however, the problem of warping (or bending deformation) is coming to be seriously considered because a warp makes the aforementioned intimate contact difficult to achieve.
It has also been known that a warp may result if a plurality of planar members having different coefficients of thermal expansion are soldered together. Solders of different kinds having different melting points generate different kinds of warps, and this effect becomes more pronounced as the melting point becomes higher. Solders with high melting points usually contain Pb as the main component. Since Pb is a soft material, it has been known to serve as a counter-measure against the problem of warping. FIG. 24 shows a power module structure of a conventional kind for a heat-generating electronic component. As can be see more clearly in FIG. 25, it is structured by soldering a heat plate 1 for contacting a heat sink (not shown) to an insulating plate 2, soldering a terminal (first terminal) 4 to this insulating plate 2, soldering a semiconductor chip 3 to the first terminal 4 so as to connect this first terminal 4 to a corresponding contact point of the semiconductor chip 3, and soldering other terminals (second and third terminals) 5 and 6 to the semiconductor chip 3 so as to connect the second and third terminals 5 and 6 each to a corresponding one of the contact points of the semiconductor chip 3. The first terminal 4 comprises an elongated main body 4A having a terminal part 4B at its tip. The heat plate 1 is adapted to be attached to the surface of the heat sink by screwing its attachment parts 7 to the heat sink surface.
In FIG. 26, symbol 1A indicates a main body part of the heat plate 1, provided with a rectangular soldering area 10-1 indicated by dotted lines over which the insulating plate 2 is attached. In FIG. 27, dotted lines 10-2 indicate another rectangular solder area on the insulating plate 2 over which the first terminal 4 is attached by soldering. In FIG. 28, dotted lines 10-3 indicate still another rectangular area defined at a center part of the main body 4A of the first terminal 4 over which the semiconductor chip 3 is attached by soldering. In FIG. 29, dotted lines 10-4 and 10-5 respectively indicate a triangular soldering area and a rectangular solder area defined on the semiconductor chip 3, respectively for attaching the second and third terminals 5 and 6 by soldering.
In the above, the soldering may be carried out by heating the heat plate, 1, the insulating plate 2, the semiconductor chip 3 and the first, second and third terminals 4, 5 and 6 as they are in the layered conditions so as to melt and harden the solder on each of the soldering areas 10-1-10-5. Alternatively, the soldering between the heat plate 1 and the insulating plate 2, that between the insulating plate 2 and the first terminal 4, that between the first terminal 4 and the semiconductor chip 3, and that between the semiconductor chip 3 and the second and third terminals 5 and 6 may be separately carried out. After a module structure 11 is thud formed by soldering, it is molded by means of a resin material 12 to obtain the power module structure shown in FIG. 24.
A mechanism for generating a warp in the insulating plate 2 and the first terminal 4 in the case of the power module structure thus formed as described above will be explained next with reference to FIGS. 30-33.
FIG. 30 shows schematically an example wherein the insulating plate 2 comprises Al2O3 and the first terminal 4 comprises Cu and is attached to the insulating plate 2 by using solder 10. The standard length of the insulating plate 2 is indicated by L1 and that of the first terminal 4 by L2. As temperature is raised from room temperature (25° C.) to Tp, the thermal expansion of the insulating plate 2 is nominal because of its material but the first terminal 4 expands outward as indicated by arrows F1, becoming longer than its standard length.
As temperature drops from Tp to Tm (>25° C.), the first terminal 4 shrinks as indicated by arrows F2 in FIG. 31. As temperature drops further from Tm, the first terminal 4 shrinks as shown by arrows F3 in FIG. 32. As temperature returns to 25° C., since the solder 10 hardens while the first terminal 4 is in an elongated condition, the first terminal 4 and the insulating plate 2 warp so as to become convex to the downward direction.
Explained more in detail, since the solder 10 has a very high melting point of about 300° C. (as compared to prior art solder with melting point of about 180° C.), the solder 10 becomes hardened at about 300° C. to connect the first terminal 4 with the insulating plate 2 when the first terminal 4 is in a thermally expanded condition while the insulating plate 2 is nearly of the original length. Thus, as temperature drops and both the first terminal 4 and the insulating plate 2 return to their original lengths, only the first terminal 4 shrinks and a warp results such that the first terminal 4 and the insulating plate 2 become convex to the downward direction.
Japanese Patent Publication Tokkai 10-167804 discloses a method of producing a circuit board for mounting to a heat-generating component such as a power module structure, characterized as using a sintered ceramic substrate bending in one direction by 1/4000- 1/100 of the length in that direction and in the perpendicular direction by ½ or less (inclusive of zero) of the bending in that direction, placing a circuit-forming metallic plate on the convex side of the ceramic substrate and a metallic plate for forming a heat-dissipating part on the concave side of the ceramic substrate and heating them to join them together such that the residual force generated at the time of producing the circuit substrate, such as when it is attached to the copper plate of the heat sink, can be increased.
With a prior art power module structure as described above, warping takes place due to the difference in the coefficient of thermal expansion, causing a gap to appear between the heat plate 1 and the heat sink, thereby adversely affecting the efficiency of heat radiation. If the power module structure is structured such that the first terminal is curved, as described above, heat is transmitted to the side where it is bent less and the solder 10 is subjected to an excessive stress. This gives rise to the problem of material fatigue.
Since the soldering area for applying solder covers the entire surface as shown by dotted lines in FIG. 27, furthermore, the deformation takes place mainly on the outer side. This means that the force corresponding to the entire length of the soldering area will be on the first terminal 4, causing it to warp.